The present invention relates to a method for continuous high-speed welding of double-ply metal tubes, in particular double-ply metal tubes that are used for brake tubes of automobiles or the like, and to a welding furnace for performing the same.
Thin band steel of for example about 0.34 mm thickness that has been subjected to copper plating of about 3 xcexcm on both sides is wound into a coil of about 2,500 to 3,000 m length and is made into a double-ply tube with a tube-making apparatus (U.S. Pat. No. 2,292,810). This is then cut into pieces of 30 to 50 m length, then 40 to 50 such pieces arranged in parallel are fed horizontally into a welding furnace from the front of the furnace, and the overlapping surfaces of the double-ply metal tubes (metal tubes having multi-ply walls) are copper brazed (Japanese Patent Publication No. S32-6111, U.S. Pat. No. 1,892,607). Then, the steps that have been performed conventionally are passing the pieces through an eddy current flaw detector to confirm the quality of the welding and whether there are defects at the tube circumference, discard defect tubes, and, if necessary, cut the pieces to suitable dimensions to obtain the product.
In this method, the tube-making speed of the tube-making apparatus is fast, at for example 150 m/min for a tube diameter of 4.76 mm, whereas the throughput speed of the welding furnace is slow at 3 to 4 m/min. Consequently, in order to keep the balance of the production steps and to rationalize the production, the tubes are cut into pieces of 30 to 50 m length, and are welded by passing them horizontally through the welding furnace, with 40 to 50 pieces arranged in parallel. In this situation, a broad space for arranging the tubes as well as operators are needed. Furthermore, in order to let the tubes advance smoothly, guide pipes, for example muffle pipes (heat-resistant pipes), are arranged inside the welding furnace, and the tubes are fed into the furnace by inserting them into the pipes. When the muffle pipes are used over extended periods of time, they bend due to thermal stress and hinder the advancement of the tubes, so that due to speed non-uniformities, defects occur in the welding. Furthermore, a space for putting down the tubes coming from the welding furnace as well as operators for collecting them and inserting them one by one into the eddy current flaw detector are needed.
The present invention provides a welding method and a welding furnace that solve these problems.
One aspect of the present invention is a method for continuous high-speed welding of double-ply metal tubes, characterized in that, using at least one neutral or reducing molten salt selected from BaCl2, mixtures of BaCl2 with MgF2, mixtures of BaCl2 with MgF2 and B2O3, and mixtures of BaCl2 with one or two or more of NaCl, MgF2 and B2O3 and Caxe2x80x94Si as a heating medium, a step of immersing a double-ply metal tube is performed while maintaining this molten salt bath at 900 to 1200xc2x0 C.
Furthermore, an aspect of the present invention is a method for continuous high-speed welding of double-ply metal tubes including a step of forming a double-ply metal tube by feeding a strip-shaped metal material that has been subjected to metal plating on both sides into a tube-making apparatus, a step of feeding the formed double-ply metal tube into a continuous heating furnace with a neutral or reducing molten salt as the heating medium, and, in the furnace, immersing a double-ply metal tube into a molten salt bath of 900 to 1200xc2x0 C. using at least one neutral or reducing molten salt selected from the group of BaCl2 and mixtures of BaCl2 with one or two or more of NaCl, MgF2, B2O3 and Caxe2x80x94Si, a step of feeding the double-ply metal tube that has left the continuous heating furnace into a high-temperature auxiliary furnace with a reducing atmosphere and eliminating a chloride thin film adhering to the circumference of the metal tube, and a step of cooling the resulting double-ply metal tube.
Moreover, an aspect of the present invention is a furnace for continuous high-speed welding of double-ply metal tubes, characterized in that downstream from a pipe-making apparatus for double-ply metal pipes, a continuous heating furnace with a neutral or reducing molten salt as a heating medium, a high-temperature auxiliary furnace filled with a reducing gas, and a cooling device with which that auxiliary furnace is furnished are provided in that order, wherein the neutral or reducing molten salt is at least one selected from the group of BaCl2 and mixtures of BaCl2 with one or two or more of NaCl, MgF2, B2O3 and Caxe2x80x94Si, and this molten salt bath is kept at 900 to 1200xc2x0 C.
The heating medium for welding furnaces that is used conventionally is reducing gas, e.g. DX gas, and its heat capacity is low. When it is replaced with a neutral or reducing molten salt in accordance with the present invention, the throughput speed of the welding furnace can be increased rapidly, because the heat capacity of the molten salt is high, improving the balance between the speeds of the tube-making apparatus and the welding furnace, and by providing the furnace with a suitable length, there is no need to cut the tubes into small pieces before feeding them into the furnace as described above, and a continuous integrated production becomes possible, with decreased factory space and reduced number of operators.
As long as the molten salt is non-oxidizing and has a large heat capacity, any kind is suitable.
Preferable are BaCl2 (melting point 962xc2x0 C., boiling point 1560xc2x0 C., specific heat capacity 0.27), a mixture of 95% BaCl2 and 5% MgF2 (melting point 962xc2x0 C., boiling point 1560xc2x0 C., specific heat capacity 0.27), a mixture of 94% BaCl2, 5% MgF2 and 1% B2O3 (melting point 1000xc2x0 C., boiling point 1560xc2x0 C., specific heat capacity 0.27), a mixture of BaCl2 with NaCl, a mixture of BaCl2 with NaCl and MgF2, a mixture of BaCl2 with NaCl, MgF2 and B2O3, a mixture of BaCl2 with NaCl, MgF2, B2O3 and Caxe2x80x94Si, and these compositions are as shown in Table 1.
Furthermore, the high-temperature auxiliary furnace filled with reducing gas is for making the tube temperature uniform and for eliminating chloride thin films adhering to the circumference of the tube when it is passed through the molten salt bath, and the cooling device is for cooling the tube.